1. Field of the Invention
The present invention relates to an inkjet recording method and apparatus and ink therefor.
2. Discussion of the Background
Recently, inkjet recording methods in which a liquid ink is discharged from a nozzle to form an image on a receiving material have rapidly been in widespread use for printers which output images prepared by computers or the like because of having the following advantages:
(1) images can be formed on plain papers;
(2) color images can be easily produced;
(3) printers can produce images at low noise; and
(4) power consumption of the inkjet recording apparatus is relatively low.
Recently, as the inkjet recording techniques have been improved, color images as good as photographs can be produced by inkjet recording methods. With the rapid progress of the inkjet recording methods, the following needs exist for the inkjet recording methods:
(1) to produce images having good water and light resistance;
(2) to produce images having good durability; and
(3) to stably discharge ink drops at a high speed.
Currently, water-soluble dyes are typically used as the colorant of inkjet inks. When a water soluble dye is used as the colorant, the resultant images have poor light resistance because water-soluble dyes typically have a poor light resistance. In addition, the resultant images tend to have a poor water resistance.
In attempting to solve the light and water resistance of ink images, aqueous inks including a pigment have been proposed because pigments have better light and water resistance than water-soluble dyes. Images recorded by pigment inks have better light and water resistance than those recorded by dye inks. However, color tone and saturation of images recorded by pigment inks are inferior to those of images recorded by dye inks. Namely, color reproducibility of the full color images formed by pigment inks is inferior to that of the images formed by dye inks. In addition, since images recorded by pigment inks have low transparency, the projected images of the images formed on an OHP (overhead projection) sheet by pigment inks has a drawback of being unclear.
For the above-mentioned reasons, pigment inks have been used only for the application in which images are formed on a special paper to produce images having good light and water resistance. Namely, pigment inks cannot record high quality images on a plain paper at a high speed.
Inkjet inks are required to have a property such that drops of the inks are stably discharged from fine nozzles of an inkjet recording head. In other words, it is needed for inkjet inks that the ink present at the exit of a nozzle does not thicken or solidify due to the evaporation of the solvent therein. With respect to the thickening and solidification, pigment inks are inferior to dye inks. In addition, the solidified pigment inks cannot be easily re-dispersed in the inks. Therefore, a clogging problem in that a nozzle of an inkjet recording head is clogged with a material such as solidified inks included in an ink tends to occur when printing is suspended for a long period of time. When printing is suspended for a while or printing a document having a long blank area, pigment inks tend to thicken due to evaporation of the solvent therein, and thereby ink drops are discharged to an undesired direction, resulting in formation of undesired images (hereinafter this problem is sometimes referred to as poor periodic discharge stability) In addition, when a pigment ink is discharged from a nozzle which has suspended to discharge the ink for a while, a problem (hereinafter referred to as a paled image problem) occurs such that the resultant images have a low density or pale color tone because the pigment content of the ink drops decreases due to deposition of the pigment.
In order to stably discharge drops of a pigment ink, the dispersibility of the pigment needs to be improved. Therefore, various surfactants and polymer dispersants have been investigated to determine whether such materials can be used as dispersants for pigment inks. In addition, self-dispersion pigments in which a hydrophilic group is incorporated on the surface of a pigment have been proposed in attempting to prepare pigment inks without a dispersant (for example, Japanese Laid-Open Patent Publications Nos (hereinafter JOPs) 10-195360 and 10-330665).
However, the periodic discharge stability of such pigment inks is not satisfactory, although the clogging problem can be fairly solved. In particular, when such pigment inks are discharged from nozzles having a small diameter, problems such that the ink discharging direction often changes and the ink discharging speed decreases occur. In addition, these inks never solve the paled image problem.
In attempting to improve the periodic discharge stability of a pigment ink, JOP 11-80639 discloses a combination technique in which an ink including a carbon black on which a hydrophilic group is grafted by a physical or chemical treatment, and one of acetylene glycol to which ethylene oxide or propylene oxide is added, polyoxyethylene alkyl ether or polyoxyethylene polyoxypropylene alkyl ether is used for a recording method in which a piezoelectric element is moved such that the ink drop is not discharged from a nozzle. However, the evaluation results of the periodic discharge stability of the ink in the system are not described therein and therefore it is not clear that the graft carbon is effective for improving the periodical discharge stability. In addition, the details of the graft carbon are not described therein. Further, whether or not the ink causes the paled image problem is not described therein.
JOP 10-95941 discloses an inkjet recording method in which an ink including a pigment capable of being dispersed without a dispersant and a specific glycol ether is contacted with a reaction liquid to form an image without blurring on a recycled paper. JOP 10-95941 also discloses a technique in which a piezoelectric element is moved (hereinafter referred to as non-discharge driving) such that the ink is not discharged to agitate the ink and to prevent the ink from thickening. However, the effect of moving the piezoelectric element is unknown judging from the examples described in JOP 10-95941.
In general, pigment inks dry and thicken such that the inks exhibit non-Newtonian flow when the pigment inks are contained in open nozzles from which drops of the inks are not discharged for a while. Namely, it is needed to always apply a driving voltage to nozzles to improve the periodic discharge stability of pigment inks, however this measure shortens the life of the recording head. In addition, JOP 10-95941 does not describe the paled image problem and therefore measures against the paled image problem are not disclosed.
With respect to the paled image problem of pigment inks, Japanese Laid-Open Ko-hyo Patent Publication 08-501330 (i.e. WO94-03546) describes that as a volatile solvent in an ink present in the vicinity of the tip edge of a nozzle evaporates, condensed products of the ink (i.e., the colorant and low volatile materials) are built up at the tip edge. Subsequently, the pigment moves toward the inside of the nozzle because the pigment has a better affinity for the ink present therein. WO94-03546 only describes that by using such an ink, the clogging problem can be solved. Namely, the paled image problem is not discussed therein.
JOP 10-279869 discloses that by using an ink having a property such that phase separation occurs at a meniscus portion of the ink in a nozzle, reliability of the inkjet apparatus can be improved. JOP 11-91096 discloses that by using such an ink and in addition by disposing of the paled ink in a nozzle in a proper amount before recording images, formation of undesired images can be prevented. However, when disposing of the paled ink, the carriage having a recording head has to be moved to a non-image forming position under which a receiving material is not present. Therefore this technique is not suitable for high speed recording. In particular, when nozzles having a small diameter are used, the ink therein is very rapidly paled. Therefore, the ink disposing operation (hereinafter sometimes referred to as an idle-discharging operation) has to be frequently performed for each nozzle which suspends to discharge the ink for a while. Therefore the technique is not practical.
JOP 2000-26779 discloses a pigment ink having a specific storage modulus and zeta potential. It discloses a pigment ink having good preservation property, however, it does not describe measures against the paled image problem and poor periodic discharge stability. When the present inventors reproduce the ink disclosed therein to evaluate the ink with respect to the paled image problem and periodic discharge stability, the ink has a poor periodic discharge stability, i.e., the ink has a drawback in that the ink in a nozzle from which ink drops are not discharged for a while is discharged in an undesired direction (an undesired discharge problem).
Because of these reasons, a need exists for an inkjet ink apparatus which can stably record high quality images without causing the undesired discharge problem and paled image problem even when the apparatus is used under various usage conditions.
Accordingly, an object of the present invention is to provide an inkjet recording method and apparatus and ink cartridge by which high quality images can be stably produced without causing the undesired discharge problem and paled image problem even when the apparatus (or the cartridge) is used under various usage conditions such that images are recorded after a long or short pause (namely, the apparatus has good periodic discharge stability).
Another object of the present invention is to provide an inkjet recording method and apparatus and ink cartridge by which high quality images can be stably recorded even when the apparatus (or the ink cartridge) uses nozzles having a small diameter.
Yet another object of the present invention is to provide an inkjet recording method and apparatus and ink cartridge by which high quality images can be stably recorded even when the images are recorded at a high speed.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by an inkjet recording apparatus including (A) a recording head including plural nozzles configured to selectively discharge a drop of an ink to form an image at an image forming area, and plural pressure generators configured to selective apply a pressure to the corresponding nozzles of the plural nozzles, from which the ink drop is discharged to form the image; (B) an ink container configured to contain the ink, wherein the ink includes a pigment, a volatile solvent (having a vapor pressure not less than 1 mmHg at 25xc2x0 C.) and a solvent having a low volatility (which has a vapor pressure less than 1 mmHg at 25xc2x0 C.), and wherein the ink has a property such that the pigment content decreases at a meniscus portion of the ink in the plural nozzles when the ink is allowed to settle; (C) a driver configured to actuate at least one of the plural pressure generators by applying a first pulse to selective discharge the ink from the discharging nozzle, wherein the driver actuates at least one of the plural pressure generators at a predetermined timing by applying a second pulse to selectively vibrate the ink at the meniscus portion to an extent such that the ink is not discharged from the nozzle (hereinafter this operation is sometimes referred to as non-discharge driving).
The ink preferably has a zeta potential not less than 20 mV (absolute value) and a property such that when the pigment content of the ink increases from 1.25 to 1.50 times the pigment content of the original ink due to evaporation of at least the volatile solvent, the absolute value of the zeta potential of the condensed ink is less than that of the original ink by at least 5 mV.
It is preferable that the volatile solvent preferably includes water, and the pigment of the ink includes particles having a surface with which an ionic group is connected optionally with a connecting group therebetween so that the pigment can be well dispersed in water.
The nozzles preferably have a diameter not greater than 25 xcexcm.
The inkjet recording apparatus preferably satisfies the following relationship:
E/wxe2x89xa65xc3x9710xe2x88x928(J/pg)
wherein w represents the weight of the ink drops discharged from the recording head to which an energy E is applied.
Each of the pressure generators preferably have a vibrating plate and an electrode opposing the vibrating plate, wherein the vibrating plate is deformed by utilizing an electrostatic force induced between the vibrating plate and the electrode to generate the pressure.
The inkjet recording apparatus preferably has a relative humidity detector configured to detect the relative humidity of the environment surrounding the inkjet recording apparatus, wherein the predetermined timing of actuating the pressure generators is determined based on the relative humidity and the pigment content decreasing property of the ink.
The actuating of the pressure generators is performed just before the ink discharging operation, wherein the number of the pulses applied during the non-discharge driving operation is from 10 to 5,000. The number of the second pulse is preferably determined based on the pause time and/or the conditions of the environment surrounding the inkjet recording apparatus.
The inkjet recording apparatus preferably has an idle-discharging mechanism configured to perform ink disposing operation at a location other than the image forming area. The idle-discharging mechanism preferably performs the ink disposing operation at an interval not less than 60 seconds.
As another aspect of the present invention, an inkjet ink is provided which includes a pigment at a first pigment content, a volatile solvent and a solvent having a low volatility and has a zeta potential not less than 20 mV (absolute value), wherein the ink further has a property such that when the pigment content of the ink increases from 1.25 to 1.50 times the first pigment content due to evaporation of at least the volatile solvent, the absolute value of the zeta potential of the condensed ink is less than that of the original ink by at least 5 mV.
It is preferable that the volatile solvent includes water, and the surface of particles of the pigment is connected with an ionic group optionally with a connecting group therebetween.
The ionic group is a cationic group selected from the group consisting of quaternary ammonium groups, quaternary alkylamine salt groups, pyridinium groups and phosphonium groups. Alternatively, the ionic group is an anionic group selected from the group consisting of xe2x80x94COOH, xe2x80x94SO3M, xe2x80x94PO3M, and xe2x80x94PO3M2, wherein M represents a hydrogen atom, an alkali metal, a quaternary ammonium group, a quaternary phosphnium group or an alkanol amine group.
As yet another aspect of the present invention, an inkjet recording ink cartridge is provided which includes at least an ink container containing an ink, wherein the ink is the inkjet ink mentioned above. The ink cartridge may further include a recording head having nozzles from which drops of the ink are discharged to form an image on a receiving material, wherein the nozzles have a diameter not greater than 25 xcexcm.
As a further aspect of the present invention, a combination of yellow, magenta, cyan and black color inks is provided each of which includes a pigment surface having a surface connected with an ionic group optionally with a connecting group therebetween, a volatile solvent and a solvent having a low volatility and which has a zeta potential not less than 20 mV, wherein the ink further has a property such that when the pigment content of the ink increases from 1.25 to 1.50 times the pigment content of the original ink due to evaporation of at least the volatile solvent, the absolute value of the zeta potential of the condensed ink is less than that of the original ink by at least 5 mV.
It is preferable that the polarity of the ionic group connected with the surface of the pigment in the black ink is different from that of the ionic groups connected with the surfaces of the pigments of the yellow, magenta and cyan inks.
As a still further aspect of the present invention, an inkjet recording method is provided which includes the steps of selectively discharging an ink from nozzles to form an image on a receiving material, wherein the ink includes a pigment, a volatile solvent and a solvent having a low volatility and has a zeta potential not less than 20 mV and a property such that the pigment content decreases at a meniscus portion of the ink in a non-selected nozzle; and vibrating the ink at the meniscus portion to an extent such that the ink is not discharged from the nozzle.
The ink preferably has a property such that when the pigment content of the ink increases from 1.25 to 1.50 times the pigment content of the original ink by evaporation of the volatile solvent, the absolute value of the zeta potential of the condensed ink is less than that of the original ink by at least 5 mV.
It is preferable that the volatile solvent preferably includes water, and the pigment of the ink has an ionic group optionally with a connecting group therebetween.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.